1. Field of the invention
The present invention relates to a novel method for the preparation of (-)9-fluoro-7-oxo-2,3-dihydro-7H-pyrido[1,2,3-de]-1,4-benzoxazine-6-carbox ylic acid derivatives having antibacterial activity and pharmaceutically acceptable salts thereof.
2. Description of the Prior Art
According to literature (Drugs of the Future 1992, 17(2), pp 559-563), it is reported that the (-) piperazine benzoxazine derivatives have a potent antibacterial activity for a wide range of bacteria.
Preparation methods for the antibacterially active compound of the formula I, one of (-)piperazine benzoxazine derivatives, are disclosed in many patents, for example, Japanese Patent Nos. 87215591 and 87198685 and European Patent No. 225552. ##STR2## wherein R, R.sub.1 and R.sub.2 each is a hydrogen or a C.sub.1 -C.sub.4 alkyl group.
These conventional methods, however, are disadvantageous in many aspects. For example, the reaction procedure employed in the conventional methods is carried out in 8 tedious steps, and a production yield is low. Further, the conventional methods use an isomer separation technique, which is unsuitable for industrial mass production.